ABSTRACT: Therapeutic oligonucleotides (e.g., small interfering RNAs (siRNAs) and antisense) hold promise as transformative drugs for the treatment of genetically defined neurodegenerative disorders, including Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). siRNAs silence disease-causing genes by targeting their cognate mRNAs for degradation, thereby preventing the expression of toxic gene products. Their inherent sequence specificity and prolonged activity provide a powerful therapeutic platform, as long as the disease is genetically defined and delivery to the relevant target tissue is achievable. However, siRNAs do not cross the blood?brain barrier and local CNS delivery by injection often results in poor retention, distribution or toxicity. Thus, efficient and non-toxic delivery represents a major hurdle in the development of RNAi-based drugs to treat neurodegenerative disorders. The goal of this proposal is to develop and characterize novel chemical scaffolds that promote simple, efficient, and non-toxic delivery of oligonucleotides and potent silencing of therapeutic targets in the central nervous system. We describe a class of fully chemically stabilized hydrophobic siRNAs (hsiRNAs) that elicit durable and potent silencing throughout the brain following bolus cerebrospinal fluid (CSF) infusion. Modifications include oligonucleotides structure, ribose, backbone and the addition of lipophilic conjugates? e.g., neuroactive steroids, endocannabinoid-like lipids, gangliosides. Extensive structure-activity relationship studies reveal that the type of conjugate defines the distribution, retention, efficacy, duration of effect, and toxicity of hsiRNA-conjugates in the central nervous system. Completion of this proposal will (i) define and characterize two novel chemical scaffolds that support potent, specific, and long-lasting silencing of target genes in the central nervous system, and (ii) validate this new platform in animal models of HD and ALS, establishing a path towards novel treatments for two neurodegenerative diseases. This proposal establishes a platform that allows direct targeting of any gene expressed in any region of the central nervous system in a rodent. Successful completion of this work will enable studies of gene function in the central nervous system and pave the way towards development of novel oligonucleotide-based therapies for genetically defined neurodegenerative diseases.